Process for electrodeposition of tin



April 30, 1946. J. w. ANDREWS PROCESS FOR ELECTRODEPOSITION OF TIN 2 Sheets-SheetI 1 Filed Nov. 19, 1941 TUA? "F 5,9/6//73 DUCT/E DEPQS/Ts 47H EMPEEHTUEE: 90 "/-T 5.5 6T N@ om@ i a5 @w w my MM H O O O O w z u m. w n

April 30, 1946. W ANDREWS 2,399,194

PROCESS FOR ELECTRODEPOSITION OF TN Filed Nov. 19, 1941 2 Sheets-Sheet 2 M4/75 OF @fz/4545 @QA/6E. 50G/f7, DUCT/E .05005/75 54TH @wf/05647065.' Q0 "E Patented Apr. 30, 1948 mocass ron mm w. Anm. mesma,

Carnegie-illinois Steel Ooi-pon tion of New Jersey annu l n mtaonarosmon or Pa.. animar to tion, aoorpora- Appueaunn ummm is. 1m, 'sa-m No.' 419,781

m accordance with the present invention there are provided certain improvements in the electrodepomtion of metals, especially the electrodeposition of tin where it is desired to obtain a thin metalv deposit or plating such as a tinplating.'

which is lustrous. highly coherent, and s a high degree of ductility.

l'lhat the electrodeposition or a metal from a. solution of its salts inmany instances results in unsatisfactory deposits is wel] known. Consequently in the deposition oi metals, particularly tin. copper. cadmium, lead and zinc. lt has been common practice to add to the electrolyte one or more compounds known as addition agents. These materials. although only minor constituents of the solution. have a very definite efl'ect on the deposit of metal obtained. Their function is to make thedeposit more-dense and adherent,

that is prevent snonsv or powdery deposits; to make smoother deposits. that is. to inhibit the natural tendency of crystal growth which results in treed or granular deposits: in some cases, to

regulate the hardne and ductility of the deposit; and in certain instances to provide a degree oi briahtness to the plated article.

The materials which have been used as addition ts comprise a wide variety of di'erent subnces. They are for the most part organic compounds oi an indennita nature-that is. they are vegetable products or extracts from vegetable ducts such gum arabic. or aloes, or animal products such as glue or gelatin, or organic waste materials such as the residues from the dismation oi' coal or wood. Such materials vary in position according to their source and even h'om batch to batch. In some cases'the exact is not denitely known. This indeiiniteness of composition makes the exact control of plating conditions dimcult and variations in re.- sults are inevitably encountered.

Ehe present invention relates to improved ad dition agents which are useful particularh1 in tinplating operations. which improved agents being denite organic chemical compounds eliminate the aforementioned diiliculties and the use of which result in the tin deposits being lustrous, substantially more ductile and substantially more coherent than those obtainable heretofore.

Theimproved addition agents of the present mvention belong to a class oi organic compounds termed sulioxides. However, only those sulioxides that can be designated by the general structural formula OH-R-SO-R/ OE where R and R." represent a benzene or a naphthalene group, may be employed.

The above formula covers the hydroxy-sulfoxides of phenol and naphthol, of which group the most easily prepared and, therefore, the most important are:

(CI. 20d-.54)

(l) The para-dihydroxy-diphenyl-sulfoxide, and

(2) The beta-dihydroxy-dinaphthyl-sulfoxide.

The remaining membersoi the group are ring@` isomers of these two compounds. It is preferred to use the phenyl compound.

The theory o! addition agents is a highly controversial subject. However. the eiect of their presence rather than the mechanism o i their acm tion is the important practical feature. Therefore. it will not be attempted to explain the action oi the improved agents except to note that on continued electrolysis, their concentration gradually diminishes, indicating` that minute amounts are consumed during the metal deposition.l

This diminution of concentration proceeds with all addition agents for some reason. However, the rate oi' depletion oi the` sulfoxides agents is much less than that of material previously used. Consequently, since less frequent replenishment is required, fluctuation oi bath compositionv is lessened and a greateruniiormity of product is obtained. 4

Moreover with many of the agents used heretofore the decrease of concentration proceeds even when the bathis standing idle. This is not true of the sulioxide agents and constitutes a markedl advantage accruing in their use.

An additional advantage of the present im proved addition agents is that they are best operated without the addition of any other addition agent to the bath. In i'act, if a lustrous, ductile,

coherent deposit is desired, the co-use of glue and similar materials with the present improved addition agents is to be avoided. However, glue may be used in conjunction with the sulfoxide agent if a "white deposit is desired; and all deposits obtained by the use of the sulfoxide agents of this invention are much more ductile agents, and depict with reasonable accuracy the behavior of any of the new agents in this and other acid tinplating baths.

In practicing the present invention the quantity of sulfoxide agent used depends to a great extent on the conditions under which it is decided to conduct the plating; that is, the acidity o f the bath. the bath temperature, and the current density. p

The relationship oi the above factors is shown in the accompanying drawings wherein: 6 F15. 1 shows graphically the relationship of than any deposit resulting from the use of glue` Fig. 1, but with high acidity.

In the` drawings, Fig. 1 shows the relationship, as has been said before, of para-dihydroxydi phenyl-sulfoxide concentration, current density, and bath temperature. Such indicated relationships hold when the plating is done from an 'agitated sulphonic acid solution containing 35 gm. per liter of tin and 62 gm. per liter of phenolsulphonic acid.

Therelationship shown by Fig. 2 obtains when the bath contains 35 gm. per liter of tin and 110 gm. per liter of phenolsulphonic acid.

assai In the drawings, curves l-I deiine the limits l at which acceptable deposits are obtained reliably when the bath temperature is 80 F.: curves 2 2 limits at 90 F.: and curves 3 3 dene the limits when the temperature is 100 F.

'I'he area between any pair of curves bearing the same reference number denes the conditions oi' current density and suli'oxlde concentration at which coherent, ductile and lustrous deposits are reliably obtained.

A study of the curves of the drawings show:

(l) For given bath composition and operating temperature the current density range over which acceptable deposits are obtained widens as the concentration of sulfoxide is increased;

(2) The effect of increasing acidity, other factors constant. is to narrow the lustrous range as will be seen by comparing Figs. 1 and 2. That is to say, the temperature increases the eifectiveness of the sulfoxide addition while increasing the acidity decreases the eectiveness of the addition.

It is to be emphasized that the curves in the drawings define the limits at which lustrous, coherent, ductile deposits can be reliably obtained. Lustrous deposits are obtainable outside the illustrated range but such attainment can not be duplicated with any degree of certainty.

The minimum concentration of the sulfoxide agents commensurate with good results is about one gram per liter. However, as shownl in Fig. 1, this minimum imposes certain limitations of plating conditions which are avoided by using larger quantities of the agent. Reference to Figs. l and 2 will show that the eect of increasing the sulfoxide agent concentration, other factors constant, is to widen the current density rangel through which acceptable deposits are obtained.

This eiIect continues until the solubility limit of the agent in the plating bath is reached; the effeet becoming less and less pronounced as this limit is approached. Increasing the concentration beyond the solubility limit while not detri mental serves no useful purpose.`

In view of the above the concentration limits may be set at: minimum-1 g./1.; maximumthe solubility limit oi' the agent which is about l0 g./l. (the exactvaluedependingon the agent. the acidity and the temperature of the bath). For most purposes, however, it is recommended that not less than' 2% g./1. be used and itis preferred touse4to5g./l. Y

The following specliic examples show the use of the improvedaddition agents of' the present invention.

ammala 1 vPlating bath composition:- 0./1. Phenolsulphonic acid 62 Stannous phenolsulphonate 138 Para-dihydroxy-diphenyl-sulphoxide 4 At room temperature this bath may be operated at a current density of anywhere from about 55 to about 165 amp. per square foot. The operating current density range can be widened and shifted upward by increasing the temperature as will be seen by reference to Fig. l.

The tin deposits produced under the above conditions are characterized by a lustre and they are more ductile and more coherent so far as is known than any of the commercial tinplating' deposits heretofore normally obtained.

Example Il Bath composition: G./l.

Sulphuric acid 40 Stannous sulphate 63 Beta dihydroxy dinaphthyl sulphoxide This bath does notY have a wide current density range, but if operated around F. and at a current density of 50-60 amp. per sq. ft., brighter tin deposits than ordinary are obtained.

Example III Bath composition: G./l. Para-cresol sulphonic acid.- 70 Stannous cresol sulphonate 145 Para dihydroxy diphenyl sulphoxide Example IV Bath composition: GJl. Hydrofluoboric acid 108 stamens iluoborate -p 86 Para-dihydroxy-diphenyl-sulphoxide 5 When this bath is thoroughly agitated and kept at F., lustrous ductile and coherent tin deposits may be obtained over a current density range of 100-200 amp./sq. ft.

Examples show the use of the improved agents in three types of acid bath, namely, two organic electrolytes, the phenol-sulphonic acid bath and the cresci-sulphonic acid bath containing, respectively, stannous phenolsulphonate and stannous cresci sulphonate, and two inorganic electrolytes, the sulfate and fluoborate. The ex- .amples are not intended as limitations but merely to substantiate the earlier statement that the new agents are applicable in all the commonly used acid tin electrolytes. It will be understood that either the beta-dihydroxy-dinaphthyl sulphoxide addition agent` or the para-dihydroxy-diphenyl sulphoxide addition agent is adapted to be used. and is intended to be used. in the foregoing examples.

We wish to call attention to the fact that the data presented in the drawings and the operating conditions recited in the examples were derived using an agitated solution. This condition was chosen since it duplicates commercial strip plating operations. and therefore represents the conditions under which the present agents will iind their widest use.

It u recognised by those 'skilled in the art oi plating that agitation is a prime iactor iniiuencing the character oi tinplate deposits. That is.

l. giftation raises the upper current density Alimit at which deposits oi optimum quality can be obtained from a given solution; the increase in current density limit depending on the degree of agitation,

2. agitation mum in improved deposits from' any given solution; the effect oi agitation appears to be independent ofthe other factors ailecting whichis the equivalent oi that prevailing when strip moves through the bath at a lineal speed of 150 ft./min. 'I'he agitation was accomplished by pumping the solution through the plating ap- -paratua It may 'be noted again for emphasis that it is common practice to use addition agents oi' many kinds in electrodepositing tin. For the most part. however, commercial addition agents are complex mixtures or compounds of indeilnite composition. Consequently, exact control oi plating conditions is diflicult to attain and variations in results inevitably are encountered.

In contra-distinction to such indenite materials the sulphoxide addition agents of the present invention are definite chemical compounds which can be prepared synthetically. They can be made to denite specications of purity. This uniformity of composition permits improved plating practices and ready duplication oi results.

I claim:

l. The process of electrodepositing tin which comprises electrolyzing an aqueous acidic tin plating solution` of a tin aryl-sulphonate in the presence of a sulphoxide addition agent in an amount not less than about 1 gm. per liter, and which is designated by the general structural formula OH-R-SO-R'OH where R and R' represent a ring selected from the group consist ing of benzene and naphthalene, the sulphoxide y addition agent being in solution.

2.. The process of producing lustrous and ductile deposits of tin which comprises electrolyzing an aqueous acid tin plating bath consisting essentially of a solution of approximately 62 gm. per liter of phenol-sulphonic acid, 135 gm. per liter of stannous phenolsulphonate and about 4 gm. per liter of para-dihydroxy-diphenylsulphomde, using a current density ofv from approximately 55 to approximately 165 amp. per square foot at room temperature, the said paradihydroxy-diphenyl sulphoxide having the structural formula to 165 amps. per square foot. an aqueous acid tin plating bath containing essentially in solution' approximately 02 lm. per liter oi phenolsuiphonis acid, 136 gm. por liter o! tin phenolsulphonate and a sulphoxide addition .agent which i's desigmated by i the general structural formula OH-R80-R'OH where R. and R' represent a ring selected from the group consisting oi benaene and naphthalene.- the 'addition agent being present in an amount oi not less than about l gm..per liter.

ductile deposits oi tin, whichV comprises electrolyzing, with a current density oi trom substantially 55 to 165 amps. per square ioot an aqueous acidic tin plating bath consisting essentially v of a solution oi' a tin salt and from 2.5 gm. per liter' 3 to approximately 5 gm. per liter oi a sulphoxide addition agent which is designated by the general R and R' represent a ring selected from they group consisting oi' benzenel and naphthalene.

5. The process oi electrodepositing lustrum, v

ductile deposits oi tin, which comprises electrolyzing an aqueous acidic tin plating bath comprising essentially a tin aromatic sulphonate and an addition agent selected from the group consisting oi para-dihydroxy-diphenyl sulphoxide and beta dihydroxy dinaphthyl sulphoxide, which addition agent is present in an amount not less than about 1 gm. per liter, and which iis 8. The process of producing lustrous, ductile,-

and adherent tin deposits, whichcomprlses electrolyzing an aqueous acidic tin plating bath consisting essentially oi a solution ofa tin salt from approximately 2.5 gm. per liter to appromfmately 5 gm. per

liter of para-dihydroxy-di- `phenyl sulphoxide as addition agent. the said' addition agent being represented' by the structural formula and using a current density of from apprommately 55 to approximately 165 amps. per square foot at room temperature.

7. In the production ci' lustrous, ductile, and

coherent tin deposits, the improvements which consist in electrolyzing an aqueous acidic stannous tin electrolyte in the presence of not less than about 1 gm. per liter of para-dihydroxydiphenyl sulphoxide as addition agent, andusing a current density of from approximately 55 to approximately amps. per square foot at room temperature, the said addition agent being represented by the structural formula s. 'nie process of eiectrpiytietin plating which comprises electrolyzing an electrolyte comprising process or producing` lustrous and.

the structural formule 10. The process of ele'ctroly'tic'tin coating" al claimed in claim 8, wherein the addition scent i; betag-dihydroxy dinaphthyl sulphcxide having .the

structural formule JOHN W. ANDREWS. 

